1. Field of the Invention
The invention relates to milling of copper with a focused ion beam, particularly to chemically-assisted milling of copper over organic dielectric materials.
2. Prior Art
The primary material used for internal wiring in advanced CMOS microchips is copper. For the debugging of these microchips with a focused-ion-beam (FIB) system, copper planes and traces should be cut uniformly and cleanly so as to electrically isolate across the separation created. Normally copper planes and traces in ICs consist of crystal grains each having a specific crystallographic orientation. Different orientations show significantly different etching rates under FIB operation. As a result, FIB etching of copper leads to a strong roughness formation on the etched copper surface which then propagates, upon perforation of the copper, to the underlying dielectric. See S. HERSCHBEIN et al., The Challenges of FIB Chip Repair & Debug Assistance in the 0.25 um Copper Interconnect Millenium, PROCEEDINGS FROM 24TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 15–19 Nov. 1998, Dallas, Tex., pp. 127–130.
The grain-dependent milling has been shown to be due to channeling of the Ga+ in certain orientations; the more densely-packed (111) crystal plane results in energy deposition near the surface and thus increased sputtering over the more open planes. See J. PHILLIPS et al., Channeling effects during focused-ion-beam micromachining of copper, J. VAC. SCI. TECHNOL. A 18(4), July/August 2000, pp. 1061–1065.
The progression of the etching process, started on the copper surface, reaches the copper/dielectric interface not simultaneously as it would if the etching were uniform, but at different points, i.e., through the crystallites with the highest etch rates. This leads to significant damage to the underlying dielectric in exposed areas yet the residual copper must still be sputtered away. This is of particular concern when milling through power planes or bus lines to expose underlying metals for device editing because the underlying dielectric might be damaged. H. XIMEN et al., Halogen-Based Selective FIB Milling for IC Probe-Point Creation and Repair, PROCEEDINGS FROM 20TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 1994, pp. 141–145.
FIG. 1 shows an example of uneven copper etching due to variations in grain crystallographic orientation. The dimension of milled box 105 is 10 μm×10 μm.
For aluminum planes and traces, compounds containing reactive species of chlorine, bromine and iodine are used to address the problem of uneven FIB milling. But halogen-containing compounds are not suitable for FIB milling of copper metallizations. While such compounds are effective to enhance milling, they spontaneously etch copper and corrode exposed copper within hundreds of microns from the initial beam exposure point. See, for example, results with halogen-based compounds including iodine-based in S. HERSCHBEIN et al., The Challenges of FIB Chip Repair & Debug Assistance in the 0.25 um Copper Interconnect Millenium, PROCEEDINGS FROM THE 24TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 15–19 Nov. 1998, Dallas, Tex., pp. 127–130, and H. BENDER et al., Investigation on Corrosion of Cu Metallization in the Focused Ion Beam System due to low I2 Background, PROCEEDINGS FROM THE 25TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 1999, pp. 135–140.
FIG. 2 shows an example of the adverse result of using iodine-based chemistry for assisted FIB milling of copper. A copper layer was milled in the circled region 205 using ethylene di-iodide enhanced metal etch chemistry as is typically used for etching of aluminum; see U.S. Pat. No. 5,840,630 of Cecere et al., FIB Etching Enhanced with 1.2 Di-Iodo-Ethane. Milling in region 205 was conducted with a 1.33 μm×0.5 μm milling box and a FIB current of 15 pA/μm2. Bright areas are corroded copper. Corrosion growth occurs on exposure to air of the hygroscopic copper iodides resulting from the etching process. Arrow 210 shows the lateral extent over which copper is affected by the enhanced etch process, in this case more than 130 μm from the milling box as indicated at 215. Corrosion of copper conductors presents serous reliability issues such as high resistivity, electrical leakage and, ultimately, conductor failure.
There is thus a need for chemical precursors for assisting the FIB milling selectivity of copper which are non-corrosive but which minimize milling of underlying dielectric layers. Chemical precursors have been proposed for FIB milling of copper over conventional dielectric materials. See U.S. Pat. No. 6,514,866 B2 and U.S. Patent Publication No. 2003/0060048 A1 of Russell et al., Chemically enhanced Focused Ion Beam Micro-Machining of Copper; U.S. Patent Publication No. 2003/0038113 A1 of Makarov et al., Process for charged particle beam micro-machining of copper; V. MAKAROV et al, Practical FIB Chemistry for Etching Copper, PROCEEDINGS OF 3RD AVS INTERNATIONAL CONFERENCE ON MICROELECTRONICS AND INTERFACES, Feb. 11–14, 2002, Santa Clara, Calif., USA, pp. 115–117; and V. MAKAROV et al., Dry Etching Considerations for Copper Metallizations, PROCEEDINGS OF THE 4TH AVS INTERNATIONAL CONFERENCE ON MICROELECTRONICS AND INTERFACES, Mar. 3–6, 2003 Santa Clara, Calif., USA, pp 198–200.
New dielectric materials, such as organic low-k dielectrics will play an increasingly significant role in advanced microelectronics. The variety of these materials increases each day. Many of these materials, especially organic ones, are extremely fragile under ion beam bombardment. See H. Bender et al., Focused Ion Beam Analysis of Organic Low-k Dielectrics, PROCEEDINGS FROM THE 26TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 12–16 Nov. 2000, Bellevue, Wash., USA, pp. 397–405.
The current state of the art is insufficient for the challenge of copper over organic dielectric. This is primarily because the art is directed to FIB milling of copper over dielectric materials which have traditionally been used in microelectronics, such as SiO2, Si3N4 and their relatives, like Fluorinated Silicon Glass (FSG). Some proposals for copper etching employ oxygen and/or oxygen containing compounds as precursors for etching of copper over these dielectrics. See U.S. Pat. No. 6,407,001 B1 of Scott, Focused Ion Beam Etching of Copper; U.S. Pat. No. 6,509,276 B2 of Scott, Focused Ion Beam Etching of Copper with Variable Pixel Spacing. 
Oxygen, water, hydrogen peroxide, ammonia carbonate, and some other precursors and their mixtures have proven successful at copper etch over traditional dielectrics. Actually the etching rate of Cu is slightly reduced by oxygen-containing precursors but that of the traditional dielectric is reduced more and thus selectivity and therefore milling uniformity at the copper dielectric interface is improved.
The effect of using oxygen-containing compounds for etching copper over organic dielectric materials is completely opposite to that for etching of copper over SiO2 dielectrics. Precursors which are favorable for milling of copper over SiO2 are detrimental for milling of copper over organic dielectric materials. One of these materials, water, is one of the best precursors for enhancing rather than retarding the milling of organic materials. See U.S. Pat. No. 6,407,001 B1 of Scott, Focused Ion Beam Etching of Copper; U.S. Pat. No. 6,509,276 B2 of Scott, Focused Ion Beam Etching of Copper with Variable Pixel Spacing; U.S. Pat. No. 5,798,529 of Wagner, Focused Ion Beam Metrology; T. STARK et al., H2O enhanced focused ion beam micromachining, J. VAC. SCI. TECHNOL. B 13(6), November/December 1995, pp. 2656–2569; U.S. Pat. No. 5,958,799 of Russell et al., Method for Water Vapor Enhanced Charged-Paricle-Beam Machining; and H. Bender et al., Focused Ion Beam Analysis of Organic Low-k Dielectrics, PROCEEDINGS FROM THE 26TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 12–16 Nov. 2000, Bellevue, Wash., USA, pp. 397–405.
Most proposed low-k dielectrics as well as all those presently used with k<3 are organic materials where carbon is one of the main elements carrying the dielectric structure. See P. HO et al., Overview on Low Dielectric Constant Materials for IC Applications, in LOW DIELECTRIC CONSTANT MATERIALS FOR IC APPLICATIONS, Ed. by P. Ho et al., Springer-Verlag, Berlin, Heidelberg 2003, Chapter I, pp. 1–21.
Another approach to FIB milling of copper over SiO2, Si3N4 and their relatives is described in U.S. Patent Publication 2001/0053605 A1 to M. PHANEUF et al., Apparatus and Method for Reducing Differential Sputter Rate. A precursor gas, tungsten hexacarbonyl, is said to dynamically produce a sacrificial layer which is intended to eliminate non-uniform milling of the surface of the material to be removed. The layer interacts with the ion-beam material-removal process to increase the uniformity of removal and is removed with the material. See also U.S. Patent Publication 2002/0195422 A1 of Sievers et al., Focused Ion Beam Process for Removal of Copper. 
The application of metal-containing precursors to provide a scattering layer was investigated and found to be beneficial in improving etch uniformity. See J. GONZALEZ et al., Chemically enhanced focused ion beam micromachining of copper, J. VAC. SCI. TECHNOL. B 19(6), November/December 2001, pp. 2539–2542. However, this approach provides no electrical isolation benefit and, even though it enhances uniformity of milling, the application of this to FIB circuit editing is clumsy at best. For example, using a tungsten deposition may achieve this sacrificial layer but removing this layer would cause the rapid degradation of organic dielectric, for reasons noted by H. Bender et al., Focused Ion Beam Analysis of Organic Low-k Dielectrics, PROCEEDINGS FROM THE 26TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 12–16 Nov. 2000, Bellevue, Wash., USA, pp. 397–405.
Methods have been proposed to improve uniformity of copper removal based on differentiation of ion dose delivery to grains with different orientation. See U.S. Pat. No. 6,509,276 B2 to Scott, Focused Ion Beam Etching of Copper with Variable Pixel Spacing, and U.S. Patent Publication No: US 2001/0053605 A1 of M. Phaneuf et al, Apparatus and Method for Reducing Differential Sputter Rates. However efficient this may be for overcoming roughness formation on copper surface, it remains clumsy and at best is only a partial solution when organic low-k is involved. Normally the ion dose delivery rate (ion current density) is defined from relative contrast of copper grains as they are seen in some secondary particles (electrons, ions, etc.) under ion bombardment. On the other hand, copper etching with an ion beam is accompanied with permanent copper grain modification (etching,. recrystallization, etc.) that therefore requires adequate redefining of the ion current density. But ion beam bombardment significantly changes relative contrast between grains and therefore confuses so the current density requires redefining. The need to minimize etching of organic dielectric is not addressed.
Three ways are known to decrease the variation in etching rates between different copper grains: (1) copper surface oxidation, (2) ion-beam bombardment of a copper surface under various (at least two) angles of incidence, and (3) ion-beam bombardment of the copper surface at lower beam energy as has been reported by J. GONZALEZ ET AL, Improvements in Focused Ion Beam Micro-machining of Interconnect Materials, J. VAC. SCI. TECHNOL. B20(6), November/December 2002, pp. 2700–2704.
For copper surface oxidation, see J. GONZALEZ et al., Improvements in Focused Ion Beam Micro-machining of Interconnect Materials, J. VAC. SCI. TECHNOL. B20(6), November/December 2002, pp. 2700–2704. Amorphous layer blocks open directions in copper grains, randomizes ion beam and reduces channeling. For ion beam bombardment of a copper surface under various angles of incidence see V. Makarov et al, Dry Etching Considerations for Copper Metallizations, Proceedings of 4th AVS International Conference on Microelectronics and Interfaces, Mar. 3–6, 2003 Santa Clara, Calif., USA, pp. 198–200. In this case grain orientation becomes insignificant because there is no single direction of bombardment, and etching of the different grains occurs with one average rate.
It is known from our experience as well as the report of H. Bender et al., Focused Ion Beam Analysis of Organic Low-k Dielectrics, PROCEEDINGS FROM THE 26TH INTERNATIONAL SYMPOSIUM FOR TESTING AND FAILURE ANALYSIS, 12–16 Nov. 2000, Bellevue, Wash., USA, pp. 397–405 that beam assisted reactions between organic low-k dielectric material and oxygen-containing precursor molecules lead to deterioration of the dielectric structure. In fact ion bombardment with no precursor results in the dielectric becoming conductive—carbonized.
No single step definitive solution has been proven which really solves the problems mentioned, so work continues for an answer that will address these issues. FIB milling of copper overlying fragile, organic dielectric materials must be made more uniform, and special measures to protect the underlying dielectric must be taken. A solution is needed which would decrease the development of roughness on FIB milled copper surfaces and provide more uniform copper etching while protecting organic dielectric from damage.